The number one reason for laptop sound to fail is for the speaker volume
to be turned down or muted. Newer laptops feature an exterior sound control
that is digitally integrated with the motherboard sound. That means when
you turn the volume up or down with the toggle on the side of the laptop,
the volume in software goes up or down as well. But older laptops, normally
with a thumb dial like the laptop motherboard to the left, don't always integrate
the volume control with the software, so your volume control in the operating
system can show that everything is fine, yet the speakers will basically
be turned off. In any case, the picture to the right shows a typical sound
card integrated with the motherboard, the connector at the top right goes
to the speakers.

The speaker in the picture to the right is a side speaker out of a Dell.
While the speakers are certainly replaceable, they are also fairly bullet
proof, or at least, getting gunked up won't make them sound much worse than
they sound to begin with. The speaker below, though tiny, reminds me more
of the old automotive replacement speakers they used to sell at Radio Shack.
At the low amplification and volumes these speakers are driven at, the amount
of junk that gets on the speaker cone doesn't really make any difference.
The two speakers below, one installed and the other in my hand, are in a
Toshiba, and are mounted cone-up just above the keyboard.

Speakers rarely fail in laptops, and the the odds of both speakers failing,
which really means a broken voice coil or broken speaker wire, are astronomical.
The first thing to do if you can't get any sound out of your system is to
check everywhere you can for a volume control that has the mute box checked
off. Depending on how good a job your laptop manufacturer did in integrating
their sound controls with the operating system, this can be a bigger chore
than you think. If the sound works for some applications and not other, it's
almost 100% sure that the problem is in software controls and not hardware.

If your sound card fails on your laptop and it's not a daughter card, it
certainly isn't worth putting in a new motherboard, even if you can find
one used. It's way too much work and you're likely to break something else,
and there are plenty of work-arounds to a dead sound card. Below is a Verbatim
USB speaker system
I
bought from Amazon for $19.64. USB speakers are really a speaker and
a sound card bundled into one. WIndows XP and Vista systems generally don't
require any software to run USB speakers, the default USB drivers work fine.
If you need a microphone, you can buy a USB mic that works the same way.
Even if you want a full functioned sound card, you don't have to replace
the motherboard. You can buy a USB sound card or a PC (PCMCIA) sound card,
either of which will probably do a better job than the original integrated
sound.

The final three screen shots are all from Windows, illustrating a few basics.
The master volume control to the right is typical for Windows based laptops
that aren't being used for recording (note there's no mic control shown).
The "mute all" box checked on the volume means you aren't going to get any
sound out of the thing, no matter what other volume controls you move around
in applications. The screen below shows what happens when you plug a USB
speaker set into a laptop with a newer version of Windows. Also not teh little
red check on the speaker icon (looks like a Rolling Stones tongue graphic).
Windows finds and installs all the necessary software in it's own driver
library. The default playback device, shown to the bottom right, becomes
the generic USB speaker driver.